If a car has multiple battery discharge problems, in my experience with an X5, the cause will most likely not be the battery or how long the car is driven. Electrical components in the X5 go bad and they consume electricity from the battery when the car is parked. I had three components go bad in a 2002 X5 3.0 which included the rain sensor, the light control module and the final stage resistor. These problems were found by the dealer checking the current draw after the car was parked. The car would also not go into sleep mode and changing the light control module solved that problem. I replaced a perfectly good one year old interstate battery and the new battery kept discharging in spite of the car being driven daily. Search on the battery drain problem and see that replacing the battery is not a cure. I wrote a thread on this subject which includes part numbers that were replaced. Now my car is working very well and thankfully the problem is solved. Hope this helps and please let me know if you have any further questions.